1. Field of the Invention
The present invention relates to bias circuits and, more particularly, to a bias circuit that is used for a high-frequency amplifier using a Hetero-junction Bipolar Transistor.
2. Description of Related Art
Recent wireless communication equipment such as a mobile phone have functions of internet connection and image transfer, and demand for high-speed transfer of large volume data is increasing. A power amplifier for transmission power amplification, which is used in a mobile phone or the like, uses a Hetero-junction Bipolar Transistor (HBT). The transmission power amplifier is required to have a low error rate for digital data, high power efficiency, high stability for control voltage variation, high stability for temperature variation and so on. A general approach to increase the stability for control voltage variation and the stability for temperature variation is to form a bias circuit on a semiconductor substrate. An example of the bias circuit is described in Japanese Unexamined Patent Publication No. 2002-9558. FIG. 4 shows the bias circuit described therein.
However, the present invention has recognized that the above bias circuit still fails to have sufficient stability for temperature variation due to a difference in the emitter size of temperature compensating transistors Q44 and Q45 and the emitter size of a bias supply transistor Q42 and an amplifier transistor Q41. Further, since a current flowing into the base of the amplifier transistor Q41 increases as a control voltage rises, a collector current of the amplifier transistor Q41 increases accordingly, which raises the heat of the amplifier transistor Q41. This can cause thermal runaway of the amplifier transistor.